An RC snubber is a fundamental building block of a switching circuit. It is generally used to control excessive peak voltage stress and EMI created during switching. For example, the RC snubber is a standard design element in many synchronous buck regulators. In such applications, uncontrolled ringing can exceed the FET breakdown voltage rating.
FIG. 1 shows a circuit model of a DrMOS (driver MOS) which includes a high-side FET (HS_FET) 110 and a low-side FET (LS_FET) 108. The parasitic elements for the LS_FET are shown. HS_FET 110 has similar parasitics (not shown). The inductors represent inductances associated with bond wires and package pins. In such applications, the ringing at the output during a switching event can cause excessive voltage over-shoot, high frequency ringing, and EMI noise radiated from the circuit. In this particular example, LS_FET 108 is rated for 25V VDSS. The Peak VDSS ring voltage across LS_FET 108, as measured at the VSWH node is 24.5 V, almost at the rated limit for the FET.
The output ringing characteristics are determined by the FET parasitic capacitance and the sum of the inductances from the package plus board inductance. The combination creates an LC resonance, with very light damping. It is common in compact form factor circuits such as DrMOS to see HF ringing approaching 150 MHz at HS_FET turn on, and 300 MHz at HS_FET turn off. This HF noise can cause the circuit to fail standards due to excessive radiated EMI.
To control output ringing, external RC snubber 104 is typically added on the PCB. Snubber 104 includes RC components coupled to the PCB as discrete components. RC snubber 104 is added to shunt the ring current away from internal path 106 through the FET and provide damping. However, in typical FET circuits, due to the mixture of internal and external parasitic impedances, the shunt is only moderately effective. In most cases, the ring voltage is still relatively close to device peak voltage rating. Thus, external snubber performance is limited by the ability to tightly couple the RC snubber across the output capacitance of the FET. This is particularly a concern in circuits with fast switching action, such as a synchronous buck regulator or motor driver. A typical solution would be to slow down turn on of the HS_FET, but this will incur the penalty of reduced circuit efficiency, and would still not reduce high-side ringing.
Thus, there is a need for a low cost and effective technique for damping the output ringing when switching power transistors.